Technological advancement of semiconductor processes toward finer design rules has made progress in realizing memory devices with higher capacities. In such a background, some solid-state memories such as flash memories are now available in place of magnetic recording media or optical recording media, which have been conventionally employed as recording media. Their example applications include IC cards or memory cards such as for digital cameras as well as secure SD (Secure Digital) cards, which have become widespread. These memory cards are increasingly demanded for higher capacities toward the future in order to record more information such as music or video images.
The memory cards are compact and thin in size. Thus, to accommodate a high-capacity memory therein, semiconductor chips are mounted by three-dimensional mounting or double-side bare chip mounting. Now, an example of such an assembly (for example, see Patent Document 1) will be described with reference to FIGS. 8A to 8G and FIG. 9.
FIGS. 8A to 8G are longitudinal sectional side views illustrating a fabrication method for dual-side mounting of semiconductor chips using the conventional flip-chip mounting technology. In addition, FIG. 9 is a longitudinal sectional front view illustrating an assembly with semiconductor chips mounted onto both surfaces of a circuit substrate using the flip-chip mounting technology. Note that in FIG. 8A to FIG. 8G and FIG. 9, like components are indicated with like symbols.
To begin with, as shown in FIG. 8A, a thermosetting adhesive 3 comprising epoxy is applied onto an upper surface 1a of a circuit substrate 1 which has a plurality of substrate electrodes 2 formed at their respective predetermined positions in the circuit on both the surfaces thereof.
At this time, the thermosetting adhesive 3 is applied so as to cover the plurality of substrate electrodes 2 on the upper surface 1a. On the other hand, as shown in FIG. 8B, a semiconductor chip 4 to be mounted has a plurality of electrode pads 7 provided on its bottom surface. On the electrode pads 7, bumps 8 are formed which melt together with the raw material thereof into an alloy to be thereby firmly fixed thereto. The semiconductor chip 4, which is securely vacuumed with a vacuum suction head 10, is fed onto the circuit substrate 1, and then pushed slightly against the thermosetting adhesive 3 with the bumps 8 aligned with the substrate electrodes 2, thereby being temporarily fixed onto the upper surface 1a via the adhesive 3 as shown in FIG. 8C.
As shown in FIG. 8D, the circuit substrate 1 having the aforementioned semiconductor chip 4 temporarily fixed to the upper surface 1a is turned upside down in the same step, and then the thermosetting adhesive 3 is applied to a lower surface 1b that faces upwardly. Subsequently, as shown in FIG. 8E, another semiconductor chip 4, which is securely vacuumed with the vacuum suction head 10, is fed onto the lower surface 1b of the circuit substrate 1. This semiconductor chip 4 is pushed slightly against the thermosetting adhesive 3 with the bumps 8 aligned with the substrate electrodes 2, thereby being temporarily fixed onto the lower surface 1b via the adhesive 3.
The circuit substrate 1 which has the semiconductor chips 4 temporarily fixed to the upper surface 1a and the lower surface 1b respectively as described above is fed to the next step. Then, as shown in FIG. 8F, the two semiconductor chips 4 temporarily fixed to the upper surface 1a and the lower surface 1b of the circuit substrate 1 are pressed under heat for approximately 30 seconds with heating and pressing heads 11 toward both the upper and lower surfaces of the circuit substrate 1. As shown in FIG. 8G, this causes the thermosetting adhesive 3 applied to both the mount surfaces 1a and 1b of the circuit substrate 1 to be simultaneously hardened under heat and contracted. This contractive force causes the two semiconductor chips 4 to be entirely pulled toward the opposing mount surfaces 1a and 1b of the circuit substrate 1, thus allowing each of the bumps 8 to be fixedly attached under pressure to the corresponding substrate electrodes 2, respectively, and thereby electrically connected thereto. In addition to this, each of the semiconductor chips 4 is firmly adhered to the mount surfaces 1a and 1b with the adhesive 3 which has been filled and hardened under heat in the entire gap between the opposing mount surfaces 1a and 1b of the circuit substrate 1. As such, the assembly is completed as shown in FIG. 9.
[Patent Document 1] Japanese Patent Laid-Open Publication No. 2003-197853